Looks like your installation got corrupted.
First step:
Try to restore to a previous restore point.
If you can't restore to a previous state:
Boot from a Windows Vista install DVD.
(if you don't have one, download an ISO file for it from some torrent network. Legal if used to replace a lost, broken or missing original DVD)
You should get an "Update" option. Use it. It will reconstruct the operating system files but will preserve all your files, installed applications and drivers.
It will remove all windows updates.
Another option:
Boot from a Linux live CD.
It will allow you to access your files and save them to some external drive or burn them to some CD/DVD.
Next, you can install any operating system you want.
Electro, February 2013
